The present invention relates generally to the field of jet drop printing and more particularly to jet drop printers of the type shown in Sweet et al, U.S. Pat. No. 3,373,437 and in Taylor et al, U.S. Pat. No. Re.28,219. In printers of this type there are one or more rows of orifices defined by an orifice plate which receive an electrically conductive recording fluid, such as for instance a water base ink, from a pressurized fluid reservoir. A fluid filament emerges from each orifice and breaks up into a drop stream.
These recorders accomplish graphic reproduction by selectively charging and deflecting the drops in each of the streams and thereafter depositing at least some of the drops on a moving web of paper or other material. The drops which are not deposited on the moving web are caught by an appropriately positioned catcher. Drop charging, deflection, and catching are all accomplished as described in the abovementioned Sweet et al and Taylor et al patents, or in Brady et al, U.S. Pat. No. 3,805,273.
One of the most difficult problems encountered in the operation of jet drop printers of the above mentioned type is that of drop stimulation. For high quality printing it is necessary that all jets be stimulated at a frequency approximating the jet natural resonance frequency. This will cause the jets to break up into streams of uniformly sized and regularly spaced drops. Furthermore it is necessary that drop generation not be accompanied by generation of satellite drops and that the break up of the streams into drops occur at a predetermined location in proximity to a charging electrode. The general concept of stimulating a jet stream to break up into drops is well known in the art and is discussed in detail in the above mentioned prior art patents.
Early attempts at stimulating array type jet drop printers, that is printers having a plurality of jet drop streams, produced unsatisfactory results due to the generation of unpredictable vibrational modes within the printing head. This problem is described in some detail in Lyon et al, U.S. Pat. No. 3,739,393, which mentions a "cusping" pattern phenomenon that is characteristic of unsatisfactory stimulation. The cusps which comprise such a pattern are observed by looking at a row of stimulated jets and noting the locations at which the jet filaments break up into streams of drops. In accordance with the Lyon et al invention, the jets are stimulated by a traveling wave technique wherein a continuous series of bending waves are caused to propagate along the orifice plate, with the waves being absorbed at the ends of the plate to prevent undesirable reflections. Each bending wave transits once past each orifice and in so doing imparts a drop stimulating disturbance to the jet associated therewith. In general these waves must be generated at a frequency near the natural frequency of the jets, and each wave, as viewed widthwise across the orifice plate, should constitute a half wave at the first order bending mode. If such waves are generated and propagated as taught by Lyon et al, then cusping patterns are eliminated, and high quality stimulation is achieved.
One problem that exists with traveling wave stimulation as taught by Lyon et al is that the bending waves are attenuated as they travel along the length of the orifice plate. This in turn causes a progressive lengthening of the jet filaments, so that filaments at the far end of the orifice plate may be considerably longer than those which are near the source of bending wave origin. This makes it difficult to locate properly the charging electrodes which, for proper operation, should be located adjacent the ends of the fluid filaments.
It is known that there is a critical filament length range, and that when the mean length of any filament falls outside this range, then the jet begins generating large numbers of satellite drops. Thus, for a given attenuation coefficient, which is determined primarily by boundary conditions around the orifice plate material, the row of jets in a jet drop printer stimulated as taught by the Lyon et al patent, may not be longer than a predetermined maximum length for successful stimulation. In an excessively long printer, the filaments at the end of the orifice plate furthest from the stimulator tend to be longer than the critical maximum and can be shortened sufficiently only be application of fairly high amplitude stimulation. If this is done, all of the filaments along the plate are shortened, so that the filaments nearest the transducer may become shorter than the critical minimum filament length.
One approach to solution of the attenuation problem encountered with traveling wave stimulation of the orifice plate is shown in Stoneburner, U.S. Pat. No. 3,882,508. Stoneburner suggests using an orifice plate having an effectively tapered width, with the plate becoming narrower toward the end furthest from the stimulation transducer. This tapering counteracts the natural tendency toward attenuation of the bending waves which travel down the length of the orifice plate. As a result of the reduction in bending wave attenuation, more uniform filament lengths are achieved and satellite drop generation is greatly reduced. The length of the orifice plate is limited, however, by acceptable maximum and minimum orifice plate widths. Additionally, some difficulty may be encountered in manufacturing such a tapered orifice plate.
Accordingly, it is seen that there is a need for a traveling wave stimulation technique in which the effect of attenuation of the waves moving along the orifice plate is substantially reduced, and in which relatively high amplitude stimulation may be applied to the orifice plate without shortening the filaments adjacent the stimulation point to a length less than the critical minimum filament length.